The present invention relates to a switching regulator for use with various kinds of electric and electronic equipment as a DC stabilizing power supply.
A switching regulator is drawing much attention as a stabilizing power supply for feeding a DC voltage and a DC current stably to a load in the form of electric or electronic equipment. While a switching regulator is small size, light weight and highly efficient, it is apt to produce noise and is complicated in construction and therefore expensive. These disadvantages are the obstacle to the extensive application of a switching regulator to common equipment. Further, a switching regulator has to be provided with a circuit for protecting it against an overcurrent which is ascribable to short-circuiting of a load connected to the switching regulator and is apt to cause a fire or damage of electronic components. Such an overcurrent protection circuit is usually constituted by a transistor and a resistor. Besides, a prior art switching regulator is provided with a switching transistor, a starting circuit having a capacitor for starting the switching transistor, an insulating transformer, a rectifying circuit for rectifying a high-frequency voltage which is generated in the secondary winding of the transformer, and output terminals connected to the rectifying circuit for delivering a DC voltage. A load is connected to the output terminals. When this kind of switching regulator is connected to a commercially available power supply, the capacitor of the starting circuit is charged and the charging current is fed to the base of the switching transistor to start the latter. A fly-back voltage is applied from the transformer to the switching transistor so that the switching voltage is generated in the secondary winding of the transformer. The high-frequency voltage is rectified by the rectifying circuit and then delivered to the load in the form of a DC voltage. In the event that a fault such as short-circuiting of the load has occurred, an overcurrent flows through a protection circuit to render a transistor thereof conductive and thereby to bypass the base current of the switching transistor. The switching transistor, therefore, stops oscillating. In this condition, the overcurrent disappears and the capacitor of the starting circuit is brought into a fully charged state. In this manner, the swtiching regulator is protected against damage due to overcurrents.
However, even if the load is restored to normal, the switching regulator having the above construction cannot bring its switching transistor to resume the oscillation immediately. Specifically, despite the absence of the overcurrent, a current is not fed to the base of the switching transistor due to the fully charged state of the capacitor of the starting circuit. The switching transistor cannot oscillating again unless a person once disconnects positively the switching regulator from the commercial AC power supply to discharge the capacitor of the starting circuit and then connects it again to the AC power supply, resulting in troublesome manipulations. Moreover, at the instant when the switching regulator is connected to an AC power supply, there is a chance of rush current which causes the protection circuit malfunctioning and thereby prevent the switching transistor from oscillating.